For the present, a conventional liquid heating device is a cast block 1 made of an alloy of zinc with aluminum. The cast block 1 has an electric heating tube 10 (or an electric heating wire) of high impedance and a water duct 11 provided in the interior. After the electric heating tube 10 is electrified and generates high temperature, the cast block 1 will conduct the high temperature to the water duct 11 to let the liquid inside heated and converted into vapor.
By the conventional liquid heating device, the process of electrify the electric heating tube 10 to generate high temperature and then having the cast block 1 conducting the high temperature to let the liquid in the water duct 11 heated or gasified may take two or three to more than ten minutes, depending on the wattage of the electric heating tube 10. However, if liquid is heated to produce vapor within two or three minutes, an electric heating tube must be of large wattage, thus consuming too much electricity. On the contrary, if the electric heating tube is of small wattage and consumes only a little electricity, it may take ten to twenty minutes to heat up the liquid into vapor, thus taking too much time and resulting in much inconvenience.
Besides, the liquid in the water duct 11 is indirectly heated by the electric heating tube 10, therefore the heat energy produced is insufficient and the heating and vaporizing of the liquid become imperfect, still having drops of water remaining at the end of the water duct 11 and not conforming to practicability.